Embodiments relate to small cell deployment within a macro cell in a wireless network.
Mobile radio frequency band(s) are both scarce and precious resources. After the inception of commercial mobile radio communication in the 1980's the numbers of subscribers have been growing exponentially. The underlying radio technology also has grown at a fast pace. In addition to conventional voice communication, data, video and real time gaming have been introduced.
These new services require a relatively higher number of bits transmitted in a unit time than conventional voice services. There are two main ways to achieve larger bit rate demands, first, efficient use of spectrum using advanced technology (based on, for example, multiple transmit and receive antennas) and, second, the use of a larger frequency band. As the frequency spectrum is already crowded the latter is often not feasible.
Introduction of the cellular concept in the 1980's allowed efficient reuse of frequency spectrums. A service area may be divided into hexagonal grids of cells which are further grouped into clusters of cells. The frequency band may be apportioned within and reused between the clusters so as to intelligently keep the co-channel interference low.
Next generation wireless technologies are based on code division multiple access (CDMA) technologies that are more robust to interference and thus universal frequency reuse or re-use of the same frequencies across cells was introduced m 2nd and 3rd generation CDMA networks.
Orthogonal frequency division multiplexing (OFDM) technology is the technique used for future 4G or International Mobile Telecommunications (IMT)-advanced networks. While OFDM is a spectrally efficient scheme and is also more suitable for multiple antenna techniques (MIMO), OFDM is more susceptible to interference. Therefore, the efficient and intelligent use of the frequency spectrum across cells is important for successful deployment of the OFDM networks.
Substantial research effort has been devoted to improve spectral efficiency, or in other words, frequency reuse of the OFDM system. Several solutions have been proposed, e.g., fractional frequency reuse (FFR) (dynamic and static), inter-cell interference coordination (ICIC) and small cell deployment (heterogeneous networks).
FFR uses a portion of the spectrum for a certain area of the cell. The portion of the spectrum is dynamically changed or allocated in a static manner. If the spectrum is dynamically allocated the uplink control signals from the surrounding cells may be used to make the allocation decisions.
In ICIC the cells periodically share some metric, for example a channel quality indicator (CQI), of a frequency band via the backhaul communication interface. The cells make the decision to allocate a frequency band from its own measurements and the information received from the surrounding cells.
Small cell deployments within a larger macro cell efficiently use the spectrum and deliver the demand for the higher bit rate in certain areas of the cell. Generally the small cells use lower transmit power to serve a small area where the demand for the service is high, or in other words, they have cell radius' of a few meters to few hundred meters. Small cells may use wireless or wired backhaul connections to the back bone network.
Indoor and outdoor pico cells, femto cells and micro cells are the main types of small cells. The categorization of the small cells are based on, for example, their transmit power levels, deployment scenarios and/or the ownership of the small cell network. If different types of small cells are deployed within a macro cell the network is also called a heterogeneous network.
FIG. 1 illustrates a conventional heterogeneous network 100. As shown, a plurality of cells 105 are arranged in a hexagonal grid of cells. Each cell may include one or more antennas 115 associated with, for example, a base station (not shown). One or more of the cells may include a plurality of small cells 115 to support services in a localized area within a cell 105.
The widely used GSM, GPRS, UMTS, HSDPA and HSUPA wireless macro cellular standards were created by the third generation partnership project (3GPP). 3GPP recently finalized the LTE standard (Release 8) and is working towards their new standards namely, release 9 and 10. Release 10 is targeted to satisfy the IMT-advanced specifications. Currently several operators around the world are planning to deploy LTE as their future macro cellular network to deliver the demand for the higher data rates.
The down link frame structure 200 of the current 3GPP LTE standard (release 8) is illustrated in FIG. 2. As shown, the down link frame structure 200 may include a physical down link control channel (PDCCH) 205 which may be associated with the first 1-3 OFDM symbols in each sub-frame. The down link frame structure 200 may include a physical control format indicator channel (PCFICH) 210 associated with the first OFDM symbol in each sub-frame.
The down link frame structure 200 may further include a physical broadcast channel (PBCH) 215 once every 10 ms and for 4 OFDM symbols. The down link frame structure 200 may further include one or more primary synchronization signals (PSS) 225 and one or more secondary synchronization signals (SSS) 220 associated with an OFDM symbol.
In the down link, the primary synchronization signal (PSS) 225, secondary synchronization signal (SSS) 220 and physical broadcast channel (PBCH) 215 may be transmitted centered on a center frequency and they occupy 6 resource blocks or 72 sub carriers. In the time domain, the PSS 225 and SSS 220 may occupy 1 OFDM symbol each while the PBCH 215 may occupy 4 OFDM symbols.
The periodicity of the PSS 225, SSS 220 and PBCH 215 may be 5 ms, 5 ms and 10 ms, respectively. The PSS 225 and SSS 220 may be used for synchronization and they may carry some cell specific sequence for cell identification as well. The PBCH 215 may carry some system information common for all users in the cell including, for example, the allocated bandwidth information. As will be recalled, the PCFICH 210 and the PDCCH 205 may occupy the entire system bandwidth and they may be transmitted in the first 1-3 symbols in every subframe.
The subframe duration may be 1 ms. The PCFICH 210 may carry the control format indicator which indicates how many symbols are used to control transmission in a subframe. The PDCCH 205 may carry the user specific control information including, for example, resource allocation information. The down link physical channels may be knotted for interference rejection. The scrambling sequence generator may be reinitialized (except for PBCH 215) every subframe based on, for example, the cell id, subframe number and a mobile identity. This may randomize the interference between cells and between mobiles.
The up link frame structure 300 of the current 3GPP LTE standard (release 8) is illustrated in FIG. 3. As shown, the up link frame structure 300 may include one or more physical uplink control channels (PUCCH) 305. Although FIG. 3 shows several other channel blocks, they are not described herein for the sake of brevity. One skilled in the art will refer to 3GPP LTE standard (release 8) for a more detailed description of the up link frame structure 300.
The LTE up link transmission (physical uplink shared channel (PUSCH) and physical uplink control channel (PUCCH) 305) may use cell specific hopping for interference averaging. The PUCCH 305 may carry the uplink control information including, for example, scheduling requests, CQI, preferred matrix index (PMI), rank information (RI), and ACK/NACK information. Multiple user control information may be code division multiplexed (CDM) and transmitted in one PUCCH 305 region.
A PUCCH 305 region may consist of two blocks. 1 Resource Block (RB)×1 slot resource units on the both side of the system bandwidth as shown in FIG. 3. Depending on the system bandwidth, the number of PUCCH 305 regions varies. For a 10 MHz bandwidth there may be 8 PUCCH 305 regions. The periodicity of the PUCCH may be configurable by the base station via the down link control signals.